6 GRAVITY, AND ITS EFFECTS. 



For this reason the particles of water north and 

 south of the equator are subject to a force or pres- 

 sure, whose direction, for any given point, is that 

 of the resultant of the centrifugal and gravitating 

 forces at that point. But since, in order that the 

 water may be at rest, it is necessary that this direc- 

 tion should be perpendicular to the liquid surface 

 of the earth, it follows that the form of this surface 

 cannot be that of a true sphere. Moreover, since 

 the centrifugal force, which, at the equator, is 

 greatest, has there also its whole effect directly 

 opposed to gravity; and since, on the other hand, 

 it not only gradually lessens in power as the 

 latitude increases, but is also less and less directly 

 opposed to gravity, it follows from this, that the 

 downward pressure of bodies at the surface of the 

 earth is least at the equator, and increases towards 

 the poles. The same result must therefore ensue 

 as you may see happen to a blown-up bladder, or 

 to a ball of india-rubber, when more strongly 

 pressed on two opposite sides than on any other 

 part, — it must become flattened towards the points 

 of greatest pressure. 



On these grounds, Newton had conjectured that 

 the liquid part of the surface of our earth was not 

 quite, but only nearly, spherical; and that, to- 

 wards the poles, it must be somewhat pressed 

 together ; and this long before his reasoning 

 could be confirmed by actual measurement. We 



